The use of acoustic resistance in transducers and sound channels is well known. In the case of a sound tube, for example, a resistance equal to its characteristic impedance will completely damp the length resonances, leaving a smooth frequency response. This is recently taught, for example, by the inventor in his chapter describing use of dampers entitled (“Earmold Design: Theory and Practice,” Proceedings of 13th Danavox Symposium, pp. 155-174, 1988). In the case of microphones and receivers, acoustic resistance can be used to smooth resonance peaks and improve the sound quality (as described by Killion and Tillman in their paper “Evaluation of High-Fidelity Hearing Aids,” J. Speech Hearing Res., V. 25, pp. 15-25, 1982). In the case of earplugs, acoustic resistance can be used in cooperation with other acoustic elements to produce high fidelity earplugs such as used by musicians in symphony orchestras (as cited in the following: Carlson, 1989, U.S. Pat. No. 4,807,612; Killion, 1989, U.S. Pat. No. 4,852,683; Killion, Stewart, Falco, and Berger, 1992, U.S. Pat. No. 5,113,967).
One problem, however, with available acoustic resistors, commonly called dampers or damping elements, is their cost. When produced with adequately tight tolerance such as to +/−20% or better, the most popular damping elements (Knowles BF-series plugs, Carlson and Mostardo, 1976, U.S. Pat. No. 3,930,560) cost $0.60 each even in very high quantities. This has been relatively stable over the life of the U.S. Pat. No. 3,930,560 and has been independent of whether the actual damping element is a cloth mesh, perforated metal (typically electroformed), or the like.
Another problem with available acoustic resistors is their design. FIG. 1 illustrates a typical early prior art acoustic resistor design. Resistor (damper) 100 is comprised of a flat piece of cloth (e.g., silk) punched into a cloth disc 101. Cloth disc 101 is mounted on a flat surface over an acoustic port or tube 103. Typically, non-corrosive rubber-like adhesive 105, for example, is used between a bottom surface of cloth disc 101 and a top surface of the structure that forms port or tube 103. Portions of the adhesive 105 typically wick into areas of the open region of cloth disc 101, as shown by reference numerals 107 and 109.
FIGS. 2A and 2B illustrate a later prior art acoustic resistor design. FIG. 2A is a side view of a damper 200, which is comprised of a flat piece of metal 203 that has perforated holes 205 in the middle. The perforated holes 205 form the open region of the damper 201. FIG. 2B is another review of the damper of FIG. 2A. As can be seen, the damper 201 is generally comprised of a perforated center section 207 (i.e., the open region) and a solid outer ring 209.
Like damper 100, damper 200 is mounted on a flat surface over an acoustic tube or port (not shown). Adhesive is likewise used between a surface of the solid outer ring 209 and a top surface of the structure that forms the tube or port. Again, portions of the adhesive wick into the perforated center section 207, partially deforming the open region of the damper 200.
In both cases, this wicking effect causes a change in the diameter of the open region of the damper, which consequently causes a change in the resistance of the damper. A 2% change in the diameter of the open region of the damper causes an approximately 4% change in the resistance of the damper. Because the diameter of the port or tube of prior art devices was typically large, however, changes in the diameter of the damper as such had at least a tolerable adverse effect on damper performance.
As the port and tube diameters of hearing improvement and audiometric devices become smaller and smaller, however, the adverse effect of adhesive wicking becomes more pronounced. In order to obtain tight tolerances of resistance values as port and tube diameters decrease, it is desirable to more tightly control the open region of the damper by eliminating adhesive wicking. On the other hand, in order to provide inexpensive assembly, adhesive is generally used. The combination of small dampers and the use of adhesive, however, causes highly variable results.
Further limitations and disadvantages of conventional and traditional systems will become apparent to one of skill in the art through comparison of such systems with the present invention set forth in the remainder of the present application with reference to the drawings.